Golf balls have undergone substantial evolution since the early days of the game. A modern golf ball 100 (see FIG. 7) is made with multiple layers, including an outer layer 102 called a “cover” and an inner body 104 called a “core.” Many modern golf balls have at least one additional layer, called a “mantle” 106, situated concentrically outside the core 104. The cover 102 is typically formed around the mantle 106 so that the cover is concentric with the mantle and the mantle and core are sealed inside the cover. For purposes of description herein, the term “core” is used generally herein to denote the portion of a golf ball located inside the cover and providing the surface on which the cover is formed, regardless of whether the core comprises one or more layers.
Conventional techniques used for forming the cover include casting, compression molding, and injection molding. The surficial dimples are formed at the same time as the cover. Injection molding is usually used for forming covers of thermoplastic materials. Compression molding is used for forming covers of polyisoprene (e.g., “balata” or gutta percha) and of certain polyurethane materials. Casting is usually used for forming covers of a thermoset material such as polyurethane, which is formed by reaction of diisocyanate, polyol, and polyamine.
Injection molding is usually performed using a mold body comprising two mold halves. Each mold half defines at least one hemispherical cavity that, when brought together with the corresponding hemispherical cavity in the other mold half, form a respective spherical ball cavity. The hemispherical cavities include retractable pins that center the core in the cavity to ensure that the cover to be formed will be concentric with the core and have substantially uniform thickness. After placing the core in the mold, the mold is closed and a liquid thermoplastic material is injected under high pressure and temperature around the core in the cavity. The pins retract into the mold halves before the thermoplastic material fully envelops the core. As the pins retract, the thermoplastic material fills the spaces previously occupied by the pins. The thermoplastic material is then allowed to cure fully and the ball is removed from the mold. Examples of injection molding of ball covers are discussed in U.S. Pat. Nos. 5,112,556 and 5,201,523.
Compression molding is performed by placing two compliant cover “blanks” around a core. Each blank is configured to become, by high-pressure molding, a respective half of the cover. The core with blanks in place is placed in a ball cavity formed by bringing together two mold halves that define respective hemispherical cavities. During molding, the mold heats (and thus softens), compresses, and urges the blanks tightly around the core at high pressure. The high pressure also seals the two blanks together around the equator of the ball. The ball is allowed to cool and then removed from the mold. See, e.g., U.S. Pat. No. 3,989,568 to Isaac and U.S. Pat. No. 3,130,102 to Watson et al.
Casting (also called “cast-molding”) is performed in a ball cavity formed by bringing together two mold halves that define respective hemispherical cavities. Casting is especially suitable for forming the cover of a thermoset material. A precise amount of liquid thermoset resin is introduced into the hemispherical cavities and partially cured (“gelled”). The core is placed in the hemispherical cavity of one mold half and supported by the partially cured resin. The second mold half is placed relative to the first mold half to enclose the core and resin in the resulting ball cavity. As the mold halves are brought together, the resin flows around the core and forms the cover. The mold body is heated briefly to cure the resin, then cooled for removal of the ball from the mold body. Advantages of casting are that it achieves substantial uniformity of cover thickness without having to use centering pins, and it can be performed at a much lower pressure inside the mold than injection molding or compression molding. Indeed, casting can be performed at substantially zero gauge pressure.
Since all three cover-molding techniques utilize, per ball, two hemispherical cavities that are brought together to form a spherical ball cavity, there is concern with events occurring at the “parting line” during molding. The parting line is represented as an equatorial line on the ball at which the two hemispherical cavities came together, more specifically where the “parting surfaces” of the opposing mold halves came together. Certain problems with the hemispherical cavities or with the parting surfaces, such as “offset” (axial mis-registration or axis-angular mismatch, roundness mismatch, or diametrical mismatch) of the hemispherical cavities with respect to each other or variations in the width of the parting line around the equator, is usually manifested as a corresponding anomaly on the ball cover formed in the mold. Example anomalies include excess equatorial “step,” excess width of flashing, excess thickness of flashing, and unequal width or thickness of flashing around the ball. Because of their adverse impact on ball trajectory during play and their objectionable appearance, these anomalies are usually removed by the manufacturer, which requires that the manufacturer include one or more post-molding manufacturing processes such as localized buffing or grinding. In general, the more pronounced the surficial anomaly, the more extensive (and costly) the post-molding buffing or grinding process. The required buffing or grinding can be of such magnitude that their effects on the ball surface are aesthetically objectionable and/or interfere with ball trajectory.
Ball-cover molds are usually used many times, and changes in the parting line can occur with repeated use of a mold. For example, axial mis-registration (side-to-side shift) or axis-angular mismatch of the mold halves with each other can occur and/or progress with repeated use of the mold. Unless strict quality control is exercised, such drifts can result in an out-of-control process that produces an unacceptable number of reject product. Unfortunately, correcting this problem usually means replacing the mold with a new one, and new cover molds are very expensive.
For manufacturing large numbers of golf balls quickly, manufacturers automate the cover-forming process as much as possible, and use cover molds configured to cover multiple golf balls simultaneously. To this end, the cover molds typically define multiple ball cavities (e.g., four or eight). To provide some correction of mis-registration of mold halves with each other, some conventional cover molds are spring-loaded. However, the resulting correction is usually not ideal for each of the multiple ball cavities defined by the mold, especially since dimensional shifts can occur in one ball cavity relative to another in the same mold.